1. Field of the Invention
The present invention relates to a step motor having a rotor which is rotatably provided in a stator and which rotates in steps according to the generated magnetic field of the stator.
2. Description of Related Art
FIG. 10 is a sectional view showing a conventional flow control valve disclosed in Japanese Patent Application Laid-Open No. 3-275979. The flow control valve has a step motor 1 serving as an actuator and a valve device 2 which controls the rate of flow of air according to the operation of the step motor 1.
The step motor 1 is equipped with: a motor case 3; a hollow columnar stator 4 provided inside the motor case 3; a rotor 5 rotatably provided in the hollow portion of the stator 4; and an actuating shaft assembly 6 which is located at the center of the rotor 5 and which reciprocates in the axial direction as the rotor 5 rotates.
The stator 4 is equipped with: a first coil 7a and a second coil 7b consisting of spirally wound conductors; first and second stator assemblies 80 and 81 which partially cover the first coil 7a; and third and fourth stator assemblies 82 and 83 which partially cover the second coil 7b.
The rotor 5 is provided with: an internal thread portion 10 formed on the inner peripheral surface thereof; a support member 9 having a spiral stopper 11 formed on an end thereof; a sleeve 12 fixed on the outer side of the support member 9; and a magnet assembly 13 arranged around the sleeve 12, the magnet assembly consisting of north-pole magnet segments and south-pole magnet segments which are alternately disposed.
The rotor 5 is rotatably supported by a first ball bearing 14 and a second ball bearing 15. The first ball bearing 14 is secured by the inner ring thereof being pressed into contact with the support member 9, while the outer ring thereof being pressed into contact with the motor case 3. The outer ring of the first ball bearing 14 is pressed against the second ball bearing 15 through a corrugated washer 16 so as to prevent the rotor 5 from moving in the axial direction. The second ball bearing 15 is secured by the inner ring thereof being pressed into contact with the a slide bearing 17 incorporated in a plate 18, while the outer ring thereof being pressed into contact with an end 19 of the sleeve 12.
The actuating shaft assembly 6 is provided with: an external thread portion 31 having a round section; a shaft 34 which has an elliptical section so as to prevent rotation; and a pin 33 which is mounted on an end of the external thread portion 31 so that it extends radially and abuts against the stopper 11 to prevent the actuating shaft assembly 6 from rotating.
The motor case 3 is secured to a housing 20, which constitutes a passage for the bypass air of an engine induction system, by a fixing screw 21 via a plate 18. The housing 20 has first to third ports 22, 23, and 24, and a two-system first bypass passage 25 and second bypass passage 26 formed therein.
The valve device 2 is equipped with: a first valve seat 27 and a second valve seat 28 which are concentric with the actuating shaft assembly 6; a first valve 29 and a second valve 30 which are secured to the actuating shaft assembly 6 and which are opposed to the first valve seat 27 and the second valve seat 28; and a spring 32 which is provided between the plate 18 and the first valve 29 and which urges the first valve 29 toward the first valve seat 27.
The operation of the flow control valve having the configuration described above will now be described.
When the first coil 7a and the second coil 7b receive an electrical signal from a control unit, not shown, the rotor 5 rotates in the forward or reverse direction according to the received electrical signal. Since the external thread portion 31 of the actuating shaft assembly 6 is screwed onto the internal thread portion 10 of the support member 9, when the rotor 5 rotates the actuating shaft assembly 6 is subjected to the elastic force of the spring 32 and reciprocates. For instance, when the actuating shaft assembly 6 moves to the right in FIG. 10, the first and second valves 29 and 30 respectively approach the first and second valve seats 27 and 28, causing the passage areas of the bypass air flowing through the bypass passages 25 and 26 to gradually decrease. When the feed amount of the actuating shaft assembly 6 reaches a maximum value, the first and second valves 29 and 30 come in contact with the first and second valve seats 27 and 28 respectively to close off the first and second bypass passages 25 and 26.
The valve closing end of the stroke of the actuating shaft assembly 6 is determined by the position where the first valve 29 comes in contact with the first valve seat 27. Moreover, the valve opening end is determined by the position where the stopper 11 on the spiral-shaped end surface at the end of the support member 9 comes in contact with the pin 33 mounted on the external thread portion 31.
In the step motor 1 having the configuration described above, the directions in which the first coil 7a and the second coil 7b are energized can be changed; a total of four energizing patterns are available. The polarity, i.e. the north pole and the south pole, of a first stator 80, a second stator 81, a third stator 82, and a fourth stator 83 is decided according to the pattern selected. With a magnetic field having these polarities, the rotor 5 rotates a predetermined angle until the magnetic forces interacting between the stators and the magnet assembly 13 are balanced, and this position is maintained.
If the energizing pattern of the first coil 7a and the second coil 7b is changed to a first energizing pattern, a second energizing pattern, a third energizing pattern, a fourth energizing pattern, the first energizing pattern, the second energizing pattern, and so on in this order, then the rotor 5 rotates in the same direction a predetermined angle and the actuating shaft assembly 6 accordingly moves a predetermined distance in the axial direction.
In order to drive the step motor 1 in exact accordance with the instructions received from the control unit, it is necessary to initialize the actuating shaft assembly 6 in advance. More specifically, the initial home position of the actuating shaft assembly 6 must be defined by moving the actuating shaft assembly 6 to the position where the stopper 11 of the support member 9 comes in contact with the pin 33.
To permit accurate initialization, a number of steps greater than the number of steps at which the actuating shaft 6 reaches the motor end is provided in the step motor 1. Thus, even when the energized first coil 7a and second coil 7b are energized to rotate the rotor 5 after the actuating shaft assembly 6 reaches the motor end position, the rotor 5 does not rotate because the stopper 11 is in contact with the pin 33, thereby preventing the actuating shaft assembly 6 from moving any further in the valve opening direction.
In the conventional flow control valve, the rotational motion of the rotor 5 is directly converted to the linear motion of the actuating shaft assembly 6. The slide bearing 17, which is slidable with respect to the elliptical shaft 34, is provided as a converting member for converting the motion of the actuating shaft assembly 6 into the axial motion. The slide bearing 17 poses a problem because the shape thereof is complicated as it has to be engaged with the plate 18 and the second ball bearing 15, thus adding to the manufacturing cost thereof.
Further, in the conventional flow control valve, the costly first ball bearing 14 must rotatably support the rotor 5, and the corrugated washer 16 must prevent the rotor 5 from moving in the axial direction in the motor case 3; these two components cannot be omitted, thus increasing cost.
Further, in the conventional flow control valve, a number of steps which is greater than the number of steps at which the actuating shaft 6 reaches the motor end is provided in the step motor 1 in order to accurately initialize the actuating shaft assembly 6, the magnetizing position of the magnet assembly 13 in relation to the circumferential position of the stopper 11 can not be determined; therefore, the magnetizing position of the magnet assembly 13 when the stopper 11 comes in contact with the pin 33 is not consistent. In other words, which of the four patterns will be adopted for energizing the first coil 7a and the second coil 7b when the actuating shaft assembly 6 is in the initial position is unknown. Accordingly, if reverse current is supplied to the first coil 7a and the second coil 7b to move the actuating shaft assembly 6 toward the valve seats 27 and 28, the pin 33 will come in contact with the stopper 11 in a plurality of steps until the energizing pattern which actually moves the actuating shaft assembly 6 is applied. This means that the relationship between the number of steps and the amount the actuating shaft assembly 6 is moved varies, posing still another problem in that an amount of flow that is supposedly controlled inevitably varies.
It is conceivable to provide a means for detecting the signal pattern for energizing the first coil 7a and the second coil 7b that indicates the magnetizing position of the magnet assembly 13 at the moment the stopper 11 touches the pin 33. Adding this means, however, would also increase the pose the cost.